The present invention relates to a semi-conductor mounting, and, more particularly to an extruded semi-conductor heat sink mounting formed of copper which incorporates a molybdenum disc.
Mounting arrangements for mounting semi-conductors have in the past included a copper mounting having a body portion and a downwardly extending stem portion. Typically, the stem portion is threaded so that it may be screwed into a threaded opening in a heat sink structure which provides the necessary cooling during operation of the semi-conductor device. The semi-conductor, typically formed of silicon, is mounted on the body portion of the mounting or, alternatively, on an upwardly extending platform portion of the mounting.
Although a copper mounting provides a highly efficient cooling mechanism for the semi-conductor, the coefficient of thermal expansion of copper is approximately 9.6.times.10.sup.-6 inches per .degree.F. while the thermal coefficient of expansion of silicon is approximately 1.6.times.10.sup.-6 inches per .degree.F. If the silicon semi-conductor were to be mounted directly upon the copper material, it would be fractured as the copper expands during the normal thermal cycle experienced during its operation. In order to prevent such fracturing, molybdenum discs have been brazed to the copper mountings and the silicon semi-conductors mounted on the molybdenum discs. Molybdenum has a thermal coefficient of expansion which is intermediate that of copper and silicon, approximately 2.7.times.10.sup.-6 inches per .degree.F., and therefore the stronger molybdenum absorbs the stress created by the thermal expansion of the copper material and prevents the semi-conductor from being damaged.
One technique which has been used for manufacturing the semi-conductor heat sink mounting has been to extrude a mounting having a body portion, a pedestal portion, and a stem portion in a single extrusion step. The copper material from which the mounting is formed is work-hardened by the extrusion process such that a relatively hard condition is reached. In order to properly affix the molybdenum disc to the copper mounting, however, it is necessary to braze the molybdenum disc into place on the pedestal portion of the mounting. The high temperature required for this brazing operation results in completely annealing the mounting, unless expensive heat treatable coppers or dispersion-hardened coppers are used. A heat sink mounting made of annealed copper is generally not acceptable because the soft annealed copper will distort as the mounting is threaded into the heat sink structure. This distortion may result in fracturing the silicon semi-conductor and the molybdenum disc upon which it is mounted.
One approach suggested by the prior art to the solution of this problem is shown in U.S. Pat. Nos. 3,197,843, issued Aug. 3, 1965, to Nippert; 3,199,000, issued Aug. 3, 1965, ti Nippert; and 3,279,039, issued Oct. 18, 1966, to Nippert, all being assigned to the assignee of the present invention. All of these patents suggest forming in a single extrusion step a work-hardened heat sink mounting for a semi-conductor after the molybdenum disc is brazed to a copper billet. The molybdenum disc and a steel weld ring are brazed onto the copper billet at the same time. A downward extending stem and an upward extending pedestal portion are then extruded from the billet simultaneously. The problem with this technique is that the molybdenum disc is subject to significant stress during the extrusion of the pedestal portion of the mounting. The disc tends to fracture or delaminate during this extrusion process because molybdenum is a very brittle material and will fracture when placed in tension. Illustrating this is the fact that Young's modulus for copper is E=16.times.10.sup.6 PSI, while Young's modulus for molybdenum is E=50.times.10.sup.6 PSI.
It is seen, therefore, that there is a need for a method of making a copper semi-conductor heat sink mounting having a stem portion, a body portion, and a pedestal portion including a molybdenum disc, in which the copper is work-hardened.